I am close to taking my long seat base, squab and cover to the upholstery shop.
I extended the angles to near the back edge for more support and was able to attach the arch between them because they had diverged enough, but had to trim the tabs to suit.
The sheet metal hump was extended by 3.5". I had to trim it a bit on one side to miss the bracket.
For screws for the tool tray slide, I decided to tap the angle reinforcement to avoid finding someone to weld on the nuts, per post #18 above. I had already drilled the full-size hole for one of them and had to use a nut on it. I used coarse screws, but would have used fine thread if I had planned better.
Because of manufacturing tolerances, the predrilled screw holes in the bracket didn't fall perfectly on the angle reinforcement. I ended up having to carve windows in the vertical leg of the angles in four places. I drilled a 17/64" hole and opened it up with this dremel bit: (I was pretty good at it by the time I got to the last one.)
https://www.dremel.com/en_US/products/-/show-product/accessories/9906-tungsten-carbide-cutter
I should have tapped holes there, too.